Hubble performance on-orbit

A summary of the Hubble Space Telescope (HST) nickel-hydrogen (NiH/sub 2/) battery performance from launch to the present. Over the life of HST vehicle configuration, charge system degradation and failures, together with thermal design limitations, have had a significant effect on the capacity of HST batteries. Changes made to the charge system configuration to protect against power system failures and to maintain battery thermal stability resulted in undercharging of the batteries. This undercharging resulted in decreased usable battery capacity as well as battery cell voltage/capacity divergence. This cell divergence was made evident during on-orbit battery capacity measurements by a relatively shallow slope of the discharge curve following the discharge knee. Early efforts to improve battery performance have been successful. On-orbit capacity measurement data indicates increases in the usable battery capacity of all six batteries as well as improvements in the battery cell voltage/capacity divergence. Additional measures have been implemented to improve battery performance, however, failures within the HST Power Control Unit (PCU) have prevented verification of battery status.     

Real time estimator for control of an orbiting single tether system

A real-time estimator is developed for the control of the Tether Dynamics Explorer (TDE) system. TDE is being used in a series of tethered satellite flight experiments whose purpose is to validate existing system models and test proposed control laws. Each experiment consists of an orbiting Delta II second stage which deploys toward the Earth a small box-shaped passive endbody at the end of a flexible tether. A discrete extended Kalman filter (DEKF) is presented which can operate in real time and in conjunction with control laws. This filter estimates the in-plane and out-of-plane tether libration angles and their rates from a proposed three-axis tether tension measurement device in the Delta second stage. The simulation results indicate that the DEKF can estimate the libration angle and their rates from a three-axis tension and length measurement with an acceptable error. The relatively simple computations required make this algorithm particularly well suited for real-time operation.<>     

Fuzzy corrections in a GPS/INS hybrid navigation system

A new concept regarding GPS/INS integration, based on artificial intelligence, i.e. adaptive neuro-fuzzy inference system (ANFIS) is presented. The GPS is used as reference during the time it is available. The data from GPS and inertial navigation system (INS) are used to build a structured knowledge base consisting of behavior of the INS in some special scenarios of vehicle motion. With the same data, the proposed fuzzy system is trained to obtain the corrected navigation data. In the absence of the GPS information, the system will perform its task only with the data from INS and with the fuzzy correction algorithm. This paper shows, using Matlab simulations, that as long as the GPS unavailability time is no longer than the previous training time and for the scenarios a priori defined, the accuracy of trained ANFIS, in absence of data from a reference navigation system, is better than the accuracy of stand-alone INS. The flexibility of model is also analyzed.     

General forms and properties of zero cross-correlation radarwaveforms

General forms for both the complementary and noncomplementary zero cross-correlation waveform sets are developed. Various properties of these codes and their relationship to zero sidelobe periodic codes are stated and proved. Some radar applications and practical considerations of using these codes are briefly discussed     

POWOW: power without wires: a SEP concept for space exploration

Electric propulsion has emerged as a cost-effective solution to a wide range of satellite applications. Deep Space 1 successfully demonstrated electric propulsion as the primary propulsion source for a satellite. The POWOW concept is a solar-electric propelled spacecraft capable of significant cargo and short trip times for traveling to Mars. It would enter aerosynchronous orbit and from there, beam power to surface installations via lasers. The concept has been developed with industrial partner expertise in high efficiency solar cells, advanced concentrator modules, innovative arrays, and high power electric propulsion systems. The latest version of the spacecraft, the technologies used, and trip times to Mars are presented. The POWOW spacecraft is a general purpose solar electric propulsion system that uses new technologies that are directly applicable to commercial and government spacecraft with power levels ranging from a LEO power level of 4 kW up to GEO spacecraft about 1 MW. The system is modular, expandable, and amenable to learning curve cost projection methods     

Demographics of principal investigators involved in NASA-funded research

This report presents the findings of a study of 2875 principal investigators identified through a database maintained by NASA's Office of Space Science and Applications. It was found that there will be a major ‘retirement wave’ involving the loss of approximately 1200 of these experienced scientists within the next 15 years. A manpower crisis can apparently only be averted by reversing the exodus from space science that occurred after the end of the Apollo programme.     

Missing radar pulse clutter processing

The degradation of conventional clutter processing filters due to missing pulses resulting from RF interference blanking, ambiguous range returns when no filter pulses are used, or eclipsing is described. The effects of a missing pulse on uncompensated and optimum clutter filters are investigated. The maximum improvement factor method and other methods are compared to the optimum filtering. New methods and results using a minimax log energy search method are described and are shown to provide performance and/or implementation complexity advantages     

Comments, with reply, on 'convergence properties of Gram-Schmidt and SMI adaptive algorithms' by K. Gerlach and F.F. Kretschmer, Jr

The commenter summarizes Q-R decomposition techniques for solving the least squares (LS) problem and comments on associated aspects of the work presented by K. Gerlach and F.F. Kretschmer, Jr. (ibid., vol.267, no.1, Jan.90). In response to the commenter's statement that the statistical properties of the LS that determine the convergence performance are well known. Gerlach and Kretschmer assert that this is true only under the assumptions that have been used in the past to analyze the convergence performance of the canceler and for only a limited number of convergence performance measures. Gerlach and Kretschmer also address the commenter's points on overmatching degrees of freedom.<>     

Coronal holes and solar magnetic fields

Since 1972, nearly continuous observations of coronal holes and their associated photospheric magnetic fields have been made using a variety of satellite and ground-based equipment. The results of comparisons of these observations are reviewed and it is demonstrated that the structure and evolution of coronal holes is basically governed by the large-scale distribution of photospheric magnetic flux. Non-polar holes form in the decaying remnants of bipolar magnetic regions in areas with a large-scale flux imbalance. There is strong indirect evidence that the magnetic field in coronal holes is always open to interplanetary space but not all open-field regions have associated coronal holes. The well-observed declining phase of the last solar cycle was characterized by stable magnetic field and coronal hole patterns which were associated with recurrent, high-speed wind streams and interplanetary magnetic field patterns at the Earth. The ascending phase of the current cycle has been characterized by transient magnetic field and coronal hole patterns which tend to occur at high solar latitudes. This shift in magnetic field and coronal hole patterns has resulted in a less obvious and more complicated association with high-speed wind streams at the Earth.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting Scientist, Kitt Peak National Observatory.     

Gravity Probe B Data Analysis

This is the first of five connected papers detailing progress on the Gravity Probe B (GP-B) Relativity Mission. GP-B, launched 20 April 2004, is a landmark physics experiment in space to test two fundamental predictions of Einstein’s general relativity theory, the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Data collection began 28 August 2004 and science operations were completed 29 September 2005. The data analysis has proven deeper than expected as a result of two mutually reinforcing complications in gyroscope performance: (1) a changing polhode path affecting the calibration of the gyroscope scale factor C _g against the aberration of starlight and (2) two larger than expected manifestations of a Newtonian gyro torque due to patch potentials on the rotor and housing. In earlier papers, we reported two methods, ‘geometric’ and ‘algebraic’, for identifying and removing the first Newtonian effect (‘misalignment torque’), and also a preliminary method of treating the second (‘roll-polhode resonance torque’). Central to the progress in both torque modeling and C _g determination has been an extended effort on “Trapped Flux Mapping” commenced in November 2006. A turning point came in August 2008 when it became possible to include a detailed history of the resonance torques into the computation. The East-West (frame-dragging) effect is now plainly visible in the processed data. The current statistical uncertainty from an analysis of 155 days of data is 5.4 marc-s/yr (～14% of the predicted effect), though it must be emphasized that this is a preliminary result requiring rigorous investigation of systematics by methods discussed in the accompanying paper by Muhlfelder et al. A covariance analysis incorporating models of the patch effect torques indicates that a 3–5% determination of frame-dragging is possible with more complete, computationally intensive data analysis.